Emergent magnetic moments produced by self-damage in plutonium

Abstract

Plutonium possesses the most complicated phase diagram in the periodic table, driven by the complexities of overlapping 5f electron orbitals. Despite the importance of the 5f electrons in defining the structure and physical properties, there is no experimental evidence that these electrons localize to form magnetic moments in pure Pu. Instead, a large temperature-independent Pauli susceptibility indicates that they form narrow conduction bands. Radiation damage from the ␣-particle decay of Pu creates numerous defects in the crystal structure, which produce a significant temperature-dependent magnetic susceptibility, (T), in both ␣-Pu and ␦-Pu (stabilized by 4.3 atomic percent Ga). This effect can be removed by thermal annealing above room temperature. By contrast, below 35 K the radiation damage is frozen in place, permitting the evolution in (T) with increasing damage to be studied systematically. This result leads to a two-component model consisting of a Curie-Weiss term and a short-ranged interaction term consistent with disorder-induced local moment models. Thus, it is shown that self-damage creates localized magnetic moments in previously nonmagnetic plutonium.disorder ͉ magnetism ͉ radiation damage A mong the interesting properties of plutonium is a complex phase diagram, which at ambient pressure exhibits six distinct solid-state phases below the melting temperature. These phases are narrowly spaced in energy, with the five lowest-energy phases separated by Ͻ2 mRyd, placing them on a molecularenergy scale as compared with the more typical 10-20 mRyd scale typical of metals such as neighboring Np and Am (1). Although there is no theoretical consensus as to the origin of the low-density fcc ␦-phase of Pu (1-7), there is an understanding that the organization of the spin and orbital moments play a key role in stabilizing this phase. The lack of significant magnetic moments is a central issue among theorists and inspired a recent experimental review (8) painstakingly describing the evidence against the existence of magnetic moments in plutonium. Recent SR ϩ studies for ␣-and ␦-phase Pu further support this absence of magnetic moments, placing the upper bound on frozen moments at 0.001 Bohr magneton ( B ) (9). However, all solid Pu phases possess large magnetic susceptibilities, suggesting they border on becoming magnetic. Consistent with this observation and general predictions of narrow 5f bands in plutonium are large electronic contributions to the specific heat in both ␣-Pu and alloy-stabilized ␦-Pu, qualifying each as a highly correlated electron system (10).Often, local magnetic moments can be induced in nearly magnetic systems by imposing a suitable perturbation. One such method is to increase disorder by introducing a low concentration of impurities via chemical substitution. For example, when very dilute quantities of Fe, Co, or Ni are doped into nonmagnetic Pd, they induce remarkably large magnetic moments by polarizing the surrounding lattice (11,12). Similarly, one-half atomic percent (at. %) Pu do...

Isochronal annealing effects on local structure, crystalline fraction, and undamaged region size of radiation damage in Ga-stabilized δ-Pu

“…It is, however, clear that the strong amount of damage is generally consistent with the magnetic susceptibility 27 and previous EXAFS results 31,43 that suggest that, in addition to the damage as measured by defect concentrations, all the atoms within a damage cascade experience a significant distortion. This result is also consistent with that expected from the displacement fields generated by the various defect structures.…”

“…We note unlike electrical resistivity measurements 28 in which the isochronal annealing curve is run twice, first to measure damage accumulated and then again to measure residual resistivity, the EXAFS isochronal annealing curve is only run one time, and in that sense is much closer to the protocol used in magnetic susceptibility isochronal annealing experiments. 27 The data presented here are improved relative to a previous EXAFS annealing study on the same 4.3 at. % Ga δ-Pu sample 31 in several respects.…”

“…65 Magnetic susceptibility, on the other hand, is induced in plutonium as a result of localized magnetic moments created by perturbations to the crystal structure, which heal and return the δ-Pu to its normal nonmagnetic state near room temperature. 27 The isochronal susceptibility experiment was closer in experimental parameters to the present one as the aged sample has the same Ga concentration and was aged for 42 days at T ≤ 30 K. 27 The resistivity experiment aged a sample with lower Ga concentration, 3.3 at. %, for 3 days at T = 20 K. 28 Although the resistivity curve plotted here for comparison was from samples held at the lowest temperature for a shorter duration to ensure the damaged areas were in the dilute limit, 28 δ-Pu held for longer times (27.7, and 38.3 days at 4.5 K) exhibited similar overall damage characteristics.…”

“…Beginning with experiment, resistivity 28 and magnetic susceptibility 27 isochronal annealing experiments were performed on cryogenically aged δ-Pu, and the fractional change in damage measured by those experiments is plotted along with data from this experiment. Resistivity depends linearly on defect concentration, and in fact, such experiments have been used in the past to estimate defect concentrations.…”

“…15,[26][27][28] Here we show the results of Pu L III -edge and Ga K-edge extended x-ray absorption fine structure (EXAFS) 29,30 experiments taken after various stages of isochronal annealing after a cryogenic aging and storage phase of 72 days. Differences in thermal behavior at various annealing temperatures are vastly reduced by collecting nearly all the data at low temperature 15 K. In this way, differences in bond length disorder as measured by the Debye-Waller factors, σ 2 , can be attributed to static bond length disorder, as opposed to differences in thermal behavior.…”